This application claims the benefit of provisional patent application No. 60/297,186, filed Jun. 8, 2001 and is a division of application Ser. No. 10/167,274, filed Jun. 10, 2002, now U.S. Pat. No. 6,758,086.
The field of the invention pertains to internal combustion engines and, in particular, to fiber optic sensors for instantaneous cylinder pressure in operating engines.
An automatic cylinder pressure sensor mounted under the hood of a vehicle is exposed to widely fluctuating temperatures both in the transducer tip area as well as in the sensor signal conditioner location. Current signal conditioner specifications require operation at −40° C. to up to 120–150° C., depending on conditioner mounting location, while the transducer tip temperature is specified in the range of −40° C. to 300–350° C. During normal continuous engine operation the transducer temperature is typically in the 150–250° C. range. During a cold engine start the transducer temperature change from −40° C. to its operating temperature occurs in a matter of several seconds.
During a vehicle lifetime, under hood temperatures over 125° C. occur relatively rarely and in special situations such as prolonged driving at maximum load (e.g., up hill towing) or in a hot climate (e.g., Arizona). In addition, the maximum temperature varies significantly depending on under hood location. Mounting near engine exhaust ports may expose a sensor signal conditioner to radiant heat in addition to ambient temperatures, whereas, in contrast locating a sensor signal conditioner near an air intake may reduce peak temperatures by as much as 20–30° C.
Unless protected and compensated, temperatures over 125° C. may result in the failure of an LED light source in the sensor signal conditioner for the fiber optic sensor. While a maximum LED storage temperature is typically 150° C., the maximum operating temperature must be lower so that the LED p-n junction temperature does not exceed 150° C. Typically, the LED current heats the junction leading to a differential between ambient and junction temperature of 10–20° C. When an LED junction reaches 150° C. rapidly growing defects result in output power reduction and potential LED failure. While in some applications an LED can be thermoelectrically cooled, in high volume automotive use the cost constraints prohibit use of any such cooling devices.